I've been using a front bag on my frame pack for 10+ years. Here is a link to 4 pictures of the bag I currently use:
The four corners of the front bag connect to the corresponding 4 corners of my frame pack. With straps and buckles it weighs less than 3 ounces. It is made of uncoated 1.9 ounce ripstop. The buckles are 5/8 adjustable quick release types. The webbing is 1/2" nylon (works more easily with buckles than 5/8"). Bag is a simple flat stuff sack with a drawstring closure and a capacity of about 500 cubic inches.
Proximal (against chest) photo shows buckles with a white 3x5 card under them for better visibility. There are additional buckles for hanging bear spray, monocular, etc.
Front bag can be used (empty or full) as an alternative to traditional shoulder straps or in addition to them. I typically use only the front bag (without shoulder straps) and last year my hiking partner decided to also do so.
I place snacks, water, monocular, hanky, bear spray, camera, caps, writing material, maps, etc. in the front bag. It is very handy to access this stuff without taking the backpack off.
Front bag does a great job of balancing the total load. It eliminates the forward straining common with backpacks.
It takes a little time to adjust to this type of system but, for me, it has been well worth it. When the front and back bags are in balance (about 4 to 1 ratio of back bag to front bag weight) I don't even need to fasten the webs going from the lower corners of the front bag to the lower corners of the frame.
Conceptually this system is like a paperboy's (or girl's) canvas delivery bag. The pack frame picks up the load, however, so all the weight is transferred to the hips instead of resting on the shoulders. In fact when I started experimenting with this system I bought a paper delivery bag so I could experiment with it.